When reservoir pressure is low in oil wells and crude oil does not flow, it is pumped for oil production. The sucker rod pump having a simple structure as shown in FIG. 8 has been used widely for many years particularly as an oil production pump method on land. This is driven by transmitting reciprocation of a reciprocating apparatus placed on the ground to the pump plunger connected to the head of a sucker rod R at the pipe bottom via this rod R.
Also in such an old pump method, for example by improving the control system of the pump jack, high speed operation is made possible to achieve high efficiency (for example, see Patent Document 1). However, in recent years, in renewal of oil well pump systems due to obsolescence, transitions to more efficient methods are advancing instead of use of such heavy mechanical pumps themselves.
From among them, as shown in FIG. 3, a method of pumping crude oil directly with a hydraulic cylinder 100 driven by hydraulic fluid from a hydraulic system 200 is mentioned. As this hydraulic system, one using a proportional solenoid piston pump that carries out direct driving by swash plate control has been developed. The proportional solenoid piston pump is a variable displacement piston pump that introduces an autogenous pressure of part of a pump discharge pressure to an operation piston as a pilot control pressure via a proportional solenoid control valve. Then, this operation piston pushes a swash plate whose tilt angle is correspondent to a discharge flow rate against a spring to control the tilt angle. In the proportional solenoid control valve, by a mechanical output generated in proportion to an excitation current as an output current responsive to a previously applied input signal, a solenoid plunger is variably displaced to control the pressure fluid acting on the operation piston. Then, via the control of the swash plate angle of the piston pump, the pressure and flow rate of the pump are controlled.
For example, as shown in the hydraulic circuit view of FIG. 4, when the flow rate and pressure are controlled via a proportional solenoid control valve 7 in proportion to an input signal (voltage or current) via a control amplifier 12, load pressure and the tilt angle of the swash plate that are correspondent to the flow rate are electrically fed back to the piston pump on the basis of detection signals from a pressure sensor 11 and a displacement rate detector 10 for variable elements. The control valve and each sensor can be structured easily as a pump body of a unit structure that mounts the piston pump 3 and its drive control system in the same housing.
In such an autogenous pressure control piston pump, when the discharge pressure becomes a pump minimum regulation pressure or below, it becomes difficult to ensure a pilot pressure for the operation piston to push the swash plate. Therefore, to ensure a control force at a low load, an external pilot method that introduces the discharge pressure of a fixed displacement pump 31 to the proportional solenoid control valve 7 as the pilot pressure force is used in many cases, as shown in the hydraulic circuit view of FIG. 5. In this case, a connection portion to an external pilot hydraulic circuit is provided on the housing of the pump body, and a pilot passage that communicates from the connection portion to a main hydraulic circuit is formed in the housing.